The production of ethers by the reaction of an isoolefin and an alcohol is a well-known commercial operation. A number of detailed descriptions of such processes, particularly as they relate to the production of methyl tert.-butyl ether (MTBE) and methyl tert.-amyl ether (MTAE) are disclosed in the prior art. These ethers are useful as high octane blending agents for gasoline motor fuels by virtue of their high Research Octane Number (RON) of about 120. Perhaps the most commonly employed reaction in the preparation of MTBE and MTAE is that between methanol and isobutylene or isoamylene, respectively. A wide variety of catalyst materials have been found to promote this reaction including ion-exchange resins such as divinylbenzene cross-linked polystyrene ion exchange resins in which the active sites are sulfuric acid groups; and inorganic heterogeneous catalysts such as boric acid, bismuth molybdate, and metal salts of phosphomolybdic acids wherein the metal is lead, antimony, tin, iron, cerium, nickel, cobalt or thorium. Also boron phosphate, blue tungsten oxide and crystalline aluminosilicates of the zeolitic molecular sieve type have also been proposed as heterogeneous catalysts for the reaction of methanol and isobutylene.
The preference for the isoalkylene-methanol reaction is in part, at least, due to the relative abundance of the starting materials. Both isobutylene and isoamylene are readily available in a petroleum refinery from both fluid catalytic crackers and as a by-product of ethylene production. Methanol is, of course, a staple commercial chemical of long standing. Moreover, isobutylene, because of its volatility, cannot be added to the gasoline pool without alkylation. Methanol cannot be added to qasoline in significant quantities because of immiscibility problems and because of its corrosiveness toward existing internal combustion engines. The combining of these two compounds thus appears to be an advantageous way to extend the gasoline pool. The modification of a gasoline by the conversion of 2-methyl-1-butene and 2-methyl-2-butene to methyl tert.-amyl ether is proposed in U.S. Pat. No. 3,482,952.
In addition to being useful in the preparation of high octane ethers for gasoline up grading, the etherification process is also useful as a separation process. The reaction of methanol with mixed C.sub.4 and C.sub.5 olefins is selective for the isobutylene and isoamylene isomers. Therefore, a mixed butylene and/or amylene stream common to refineries can use the aforesaid etherification process to separate this mixture and to produce a stream of essentially pure normal butenes and/or amylenes and essentially pure MTBE and/or MTAE. The ethers can subsequently be cracked to produce essentially pure isoalkylenes.
A wide variety of reaction conditions have heretofore been proposed for carrying out the reaction of isobutylene or isoamylene with methanol, depending in part upon the type of catalyst employed in each case. Thus, both vapor phase and liquid phase processes are known in which reaction temperatures are from about 50.degree. C. to about 400.degree. C., pressures vary from atmospheric to 1,500 psig, and the mole ratios of methanol to isoalkylene range from 0.1:1.0 to about 10:1. Both batch type and continuous process schemes are said to be suitably employed.
It is commonly the case that the source of isobutylene is a mixed C.sub.4 hydrocarbon stream from a refinery operation, and the reaction with methanol is carried out in the liquid phase at a temperature not exceeding 100.degree. C. The quantity of the MTBE produced depends upon the isobutylene content of the C.sub.4 hydrocarbon stream used. When a C.sub.4 hydrocarbon stream cut from steam cracking is used, providing a feedstock with approximately 50% isobutylene after butadiene extraction, the reactor effluent can contain almost 60% MTBE and can sometimes be used as a gasoline component without further treating. It is generally more desirable, however, to separate the unreacted C.sub.4 's from the reactor effluent by distilling off the unconverted C.sub.4 's. When this is done, MTBE of about 98% purity can be produced at an isobutylene conversion of about 95%. A further increase in the conversion, based on isobutylene, can be achieved only by usinq a higher methanol/isobutylene ratio in the reactor feedstock. Because greater than stoichiometric amounts of methanol are used in the high conversion MTBE processes (also to allow for fluctuating isobutylene concentrations), additional steps have to be included in such processes to recover the excess methanol from reactor effluent. The recovered methanol is then recycled to the reactor feed stream.
In the MTBE reactor, one of the more significant side reactions which occurs is the dehydration of methanol to form dimethyl ether. The formation of dimethyl ether is particularly undesirable since it is a detrimental catalyst poison in down stream processing of any unreacted hydrocarbons, such as normal C.sub.4 -C.sub.5 olefins and C.sub.4 -C.sub.5 paraffins, which are present in the hydrocarbon feed to the MTBE reactor.
It has heretofore been proposed to treat the effluent from the MTBE reactor in fractionation column and recovering the MTBE as the bottoms product. The light hydrocarbons, methanol and dimethyl ether are recovered as the column overhead and then passed through a first adsorption system to selectively adsorb the methanol and thence through a second adsorption system to isolate the dimethyl ether. The methanol adsorption system is regenerated, advantageously using a purge stream which is the intended hydrocarbon feed for the MTBE reactor, and the effluent methanol-containing hydrocarbon stream recycled to the MTBE reactor. The dimethyl ether adsorption system is regenerated by purge desorption and the effluent purge stream vented from the system for disposal or other treatment.